1. Field of the Invention
The present invention relates to a sheet feeder which feeds, from a storage case in which a plurality of sheets are stored, the sheets one-sheet by one-sheet, and the invention also relates to an image forming apparatus having the sheet feeder.
2. Description of the Related Art
When an image forming apparatus is used under a high humidity environment, transfer sheets stored in the apparatus absorbs humidity. Since transfer sheets which absorb humidity in a state where they are stacked in the apparatus absorb each other, there is a fear that sheet feeding failure is caused. Further, even if sheets can be fed, since insulation resistance is largely deteriorated, transferring ability of toner image is deteriorated and a failure image is prone to be generated.
To prevent a sheet from absorbing humidity, it is conventionally proposed a method in which a peripheral temperature of transfer sheets is increased to prevent the transfer sheets from absorbing humidity. As this method, there is a method in which under the high humidity environment, a dehumidification heater for increasing a temperature of a tray on which transfer sheets are stacked or a temperature of air in the apparatus is provided, thereby preventing the transfer sheets from absorbing humidity.
According to such a method, it is necessary to pass a current through the dehumidification heater and it is waste of electricity. Thus, there is also proposed to use a dehumidification unit including chemical absorbent (Japanese Patent Application Laid-open No. 9-44063).
There is also proposed a technique for conditioning humidity using a dehumidification member which can be thermally regenerated. According to this method, a desiccant type dehumidifier in which the dehumidification member absorbs humidity, the dehumidification member is heated and dried so that absorbed humidity is removed and the dehumidification member is regenerated is disposed in an image forming apparatus (Japanese Patent Application Laid-open No. 2005-77762).
According to the method using the dehumidification heater, however, it takes time to warm the peripheral environment of transfer sheets. In addition, even if the peripheral environment is warmed once and humidity of the transfer sheets is removed, if the sheet storage case is pulled out for adding transfer sheets or the like, outside air having high humidity enters, and it also takes time for again removing humidity from the transfer sheets which absorbed humidity. Therefore, there is a problem that it takes time before sheet feeding operation is started.
The method in which the desiccant type dehumidifier is incorporated in the image forming apparatus also has a problem that the dehumidification operation can not be started immediately before the dehumidification member is thermally regenerated, and it takes time for dehumidifying the transfer sheets.